A junction field effect transistor (JFET) or a static induction transistor (SIT) constituting a power conversion device is a power semiconductor switching element capable of achieving high-speed operation in high-voltage/high power region.
The power semiconductor switching element may be typified by a normally-on characteristic in which drain current flows therein when the gate voltage is 0 [V]. When drain voltage is applied in a state where negative polarity voltage is not sufficiently applied to a gate electrode, large drain current may flow to break the power semiconductor switching element. Therefore, the power semiconductor switching element is comparatively more difficult to handle than a transistor having a normally-off characteristic, such as a bipolar transistor, a metal-oxide semiconductor field-effect transistor (MOSFET), or an insulated gate bipolar transistor (IGBT).
To solve such a technical problem, a power conversion device is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2001-251846, the entire content of which is incorporated herein by reference. The power conversion device constituted by a normally-off composite semiconductor element (hereinafter, referred to merely as “cascode element”) in which a static induction transistor (SIT) and an insulated gate field-effect transistor (IGFET) are cascode-connected to each other is proposed.
FIG. 4 is a main circuit diagram of a bridge-connected power conversion device in which some power semiconductors are replaced by other ones.
That is, in the power conversion device of FIG. 4, a normally-on semiconductor element is changed from a static induction transistor (SIT) to a junction field effect transistor (JFET) 111, and an inverter main circuit 3 of the power conversion device includes six three-phase arms connected in a bridge configuration, wherein an arm of each phase is constituted by a cascode element 110 including the junction field effect transistor 111 and a metal-oxide semiconductor field-effect transistor (MOSFET) 112 of a normally-off semiconductor element which are electrically connected in series to each other.
In FIG. 4, reference numeral “1” denotes a DC power supply, “2” denotes a smoothing capacitor, “113” denotes a diode (rectifier diode) parasitic (incorporated) between the source and the drain regions at the manufacturing time of the metal-oxide semiconductor field-effect transistor 112.
In the conventional inverter main circuit 3 having the above configuration, the cascode element 110 is put into an OFF state in a gate power at the loss of gate power supply occurring, e.g., at power-on time or at abnormal time, so that it is possible to prevent a short circuit fault of the inverter main circuit 3. Further, a gate driving circuit (not illustrated) is connected to the gate terminal of the metal-oxide semiconductor field-effect transistor 112 to thereby switch ON/OFF of the cascode element 110.
As described above, the diode (rectifier diode) 113 is incorporated between the source and the drain regions of the metal-oxide semiconductor field-effect transistor 112. Thus, with attention focused on the U-phase, current (hereinafter, referred to merely as “return current”) can be made to flow from an output terminal U (common connection terminal shared with a negative arm that is the X-phase arm) to a positive-side DC bus 1p through a diode 113u and a junction field effect transistor 111u. 
The following points have not been addressed in the power conversion device of FIG. 4.
That is, when the cascode element 110u of one arm (e.g., U-phase arm) is turned conductive (ON) at the time of switching operation, a diode 113x of a metal-oxide semiconductor field-effect transistor 112x of the counterpart arm (X-phase arm) is turned non-conductive (OFF).
At this time, minority carriers are accumulated in a depletion layer generated in a PN junction part of the diode 113 in a non-conductive state. The minority carriers accumulated in the depletion layer flow to the diode 113x as reverse recovery current, causing reverse recovery loss. The reverse recovery loss is switching loss of the diode 113x and is caused every switching operation. Further, the reverse recovery current flows into the cascode element 110u in the conductive transient state, as a result, the switching loss of the cascode element 110u increases.
The increase in the switching loss results in an increase in heat generation loss. This requires the use of a large size cooling heat sink, leading to an increase in the size of the power conversion device.
Such a problem is not specific to the junction field effect transistor 111 constituting the cascode element 110 in the power conversion device but occurs also in the case where the junction field effect transistor 111 is replaced by the static induction transistor (SIT).
The present invention has been made to solve the above problem, and an object thereof is to provide a power conversion device capable of reducing the switching loss and the heat generation loss caused by the reverse recovery current.